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The fear of the number 13 has been given a scientific name: "triskaidekaphobia"; and on analogy to this the fear of Friday the 13th is called paraskevidekatriaphobia, from the Greek words Paraskeví (Παρασκευή, meaning "Friday"), and dekatreís (δεκατρείς, meaning "thirteen").[1]

The superstition surrounding this day may have arisen in the Middle Ages, "originating from the story of Jesus' last supper and crucifixion" in which there were 13 individuals present in the Upper Room on the 13th of Nisan Maundy Thursday, the night before his death on Good Friday.[2][3] While there is evidence of both Friday[4] and the number 13 being considered unlucky, there is no record of the two items being referred to as especially unlucky in conjunction before the 19th century.[5][6][7]

He [Rossini] was surrounded to the last by admiring friends; and if it be true that, like so many Italians, he regarded Fridays as an unlucky day and thirteen as an unlucky number, it is remarkable that on Friday 13th of November he passed away.[8]

It is possible that the publication in 1907 of Thomas W. Lawson's popular novel Friday, the Thirteenth,[9] contributed to disseminating the superstition. In the novel, an unscrupulous broker takes advantage of the superstition to create a Wall Street panic on a Friday the 13th.[5]

In Spanish-speaking countries, instead of Friday, Tuesday the 13th (martes trece) is considered a day of bad luck.[12] The Greeks also consider Tuesday (and especially the 13th) an unlucky day[citation needed]. Tuesday is considered dominated by the influence of Ares, the god of war. A connection can be seen in the etymology of the name in some European languages (Mardi in French or martes in Spanish). The fall of Constantinople to the Fourth Crusade occurred on Tuesday, April 13, 1204, and the Fall of Constantinople to the Ottomans happened on Tuesday, 29 May 1453, events that strengthen the superstition about Tuesday. In addition, in Greek the name of the day is Triti (Τρίτη) meaning literally the third (day of the week), adding weight to the superstition, since bad luck is said to "come in threes".[dubious– discuss]

In Italian popular culture, Friday the 17th (and not the 13th) is considered a day of bad luck.[13] The origin of this belief could be traced in the writing of number 17, in Roman numerals: XVII. By shuffling the digits of the number one can easily get the word VIXI ("I have lived", implying death in the present), an omen of bad luck.[14] In fact, in Italy, 13 is generally considered a lucky number.[15] However, due to Americanization, young people consider Friday the 13th unlucky as well.[16]

According to the Stress Management Center and Phobia Institute in Asheville, North Carolina, an estimated 17 to 21 million people in the United States are affected by a fear of this day, making it the most feared day and date in history. Some people are so paralyzed by fear that they avoid their normal routines in doing business, taking flights or even getting out of bed. "It's been estimated that [US]$800 or $900 million is lost in business on this day".[6] Despite this, representatives for both Delta Air Lines and now-defunct Continental Airlines have stated that their airlines do not suffer from any noticeable drop in travel on those Fridays.[17]

In Finland, a consortium of governmental and nongovernmental organizations led by the Ministry of Social Affairs and Health promotes the National Accident Day, which always falls on a Friday 13th.[18]

A study in the British Medical Journal, published in 1993, concluded that there "is a significant level of traffic-related incidences on Friday the 13th as opposed to a random day, such as Friday the 6th, in the UK."[19] However, the Dutch Centre for Insurance Statistics (CVS) on 12 June 2008 stated that "fewer accidents and reports of fire and theft occur when the 13th of the month falls on a Friday than on other Fridays, because people are preventatively more careful or just stay home. Statistically speaking, driving is slightly safer on Friday the 13th, at least in the Netherlands; in the last two years, Dutch insurers received reports of an average 7,800 traffic accidents each Friday; but the average figure when the 13th fell on a Friday was just 7,500."[20][21]

This sequence, given here for 1900–2099, follows a 28-year cycle from 1 March 1900 to 28 February 2100. The months with a Friday the 13th are determined by the Dominical letter (G, F, GF, etc.) of the year. Any month that starts on a Sunday contains a Friday the 13th, and there is at least one Friday the 13th in every calendar year. There can be as many as three Friday the 13ths in a single calendar year; either in February, March and November in a common year starting on Thursday (such as 2009, 2015 or 2026) (D), or January, April and July in a leap year starting on Sunday (such as 2012) (AG).

The longest period that can occur without a Friday the 13th is fourteen months, either from July to September the following year being a common year starting on Tuesday (e.g., between 2001–02, 2012–13, and 2018–19), or from August to October the following year being a leap year starting on Saturday (e.g., between 1999–2000 or 2027–28).

Each Gregorian 400-year cycle contains 146,097 days (365 × 400 = 146,000 normal days, plus 97 leap days). 146,097 days ÷ 7 days per week = 20,871 weeks. Thus, each cycle contains the same pattern of days of the week (and thus the same pattern of Fridays that are on the 13th). The 13th day of the month is slightly more likely to be a Friday than any other day of the week.[22][23] On average, there is a Friday the 13th once every 212.35 days (compared to Thursday the 13th, which occurs only once every 213.59 days).

^Hartston, Willam (1 June 2007). Encyclopedia of Useless Information. Sourcebooks, Incorporated. p. 365. ISBN9781402248382. In Christian tradition, fear of Friday the 13th stems from the day of the Crucifixion (Friday) and the number at the Last Supper (13). Despite these origins, the Friday the 13th superstition dates back only to the Middle Ages.|accessdate= requires |url= (help)

^Friday has been considered an unlucky day to undertake journeys or begin new projects at least since the 14th century, as witnessed by Chaucer's Canterbury Tales.